It's only when something is lacking that we begin to analyze and contemplate what that thing actually isSo, if we're even asking the question, “What is love?” it probably means that we don't feel completely loved, or that someone doesn't feel completely loved by us.

My definition would be when you want to be in someone's company and you can't go a day without thinking about that certain thing, and without it you feel like you're not so complete. But that's more the definition of being in love.
Loving something is simply when you want to continue having that thing and don't want to lose it.